1. Field of the Invention
This invention relates to the field of battery charging circuits, and particularly to methods of blocking current flow from a charged battery when the charging circuit is not in use.
2. Description of the Related Art
A Conventional battery charging circuit is shown in FIG. 1. A charger supply voltage Vsupply is provided to the charging circuit at a charger voltage input terminal 8. The charging circuit provides a charging current to a rechargeable battery 10 connected to a battery connection terminal 12; the voltage across the battery is designated Vbat. A pass device, typically a PNP transistor Q1, is connected between Vsupply and battery connection terminal 12, and conducts the charging current in response to a control signal applied at Q1""s base. The charging circuit typically includes an isolation diode 14 connected in series between Vsupply and Q1 (with the voltage on the cathode side of diode 14 being xe2x80x9cVchgxe2x80x9d), and a blocking diode 16 connected in series between Q1 and battery connection terminal 12. Control circuits such as a charge current sense circuit 18, a battery voltage sense circuit 20, and a pass device drive circuit 22 complete the charging circuit.
Isolation diode 14 is necessary to prevent damage to the charging circuit in case a voltage having a reverse polarity is inadvertently applied to charger voltage input terminal 8, and to prevent Vbat from being present on the input terminal when Vsupply is not present.
Blocking diode 16 is needed to prevent reverse conduction of the pass device. Reverse conduction could occur, for example, if the charging voltage never exceeds the battery voltage. If this occurs, battery voltage Vbat, through the parasitic base-collector diode of a PNP pass device (or body diode of a MOSFET pass device), could bias up the charger control circuits; thereby gradually draining the battery.
A conventional battery charging circuit also typically includes a means of holding the pass device off when the charger is not in use. This may be accomplished, for example, by connecting a resistor 24 or a switch between Q1""s base and emitter. There are several situations in which it is important to hold Q1 off when the charger is not in use. For example, in some applicationsxe2x80x94charging a lithium ion (Li+) battery, for examplexe2x80x94when the charger must not continue to provide current to the battery after charging is complete. The pass device is held off to effect this. Also, the charger must not attempt to charge a battery when Vchg less than Vbat. Isolation diode 14 would prevent Vbat from appearing on charger voltage input terminal 8, but the system should also hold Q1 off to prevent control circuits on the Vsupply side of the charging circuit from draining the battery. This is also true if Vsupply is not present at all.
The configuration shown in FIG. 1 suffers from several drawbacks, however. The charging circuit requires two diodes, which can be costly. In addition, Vsupply must be at least two diode drops above Vbat to maintain charging. This increases power consumption, and forces Vsupply to be at a higher voltage than might be desirable.
A system and method for battery isolation in a charging system is presented, which overcomes the problems noted above. The invention enables the charger circuit""s current consumption to be zero when not in use, eliminates the need for a blocking diode, and reduces power consumption and charger supply voltage required to maintain charging.
The present battery charging system and method include a charger voltage input terminal for connection to a charger supply voltage Vsupply, an isolation diode connected to the charger voltage input terminal, and a PNP pass transistor connected in series between the isolation diode and a battery connection terminal and which conducts a charging current in response to a drive signal applied to its base; the pass transistor side of the diode is at a voltage Vchg. The current charges a battery connected to the battery connection terminal; the voltage across the battery is designated Vbat.
The system also includes a first switch arranged to couple the pass transistor""s base to Vchg when closed in response to a first control signal, and a second switch arranged to couple the base to Vbat when closed in response to a second control signal. A controller provides the first and second control signals to the switches. The first switch is closed and the second switch is opened when Vchg greater than Vbat, such that the pass transistor""s base-collector junction blocks current from a charger supply voltage from flowing through the pass transistor when the charger is not in use. The second switch is closed and the first switch is opened when Vbat greater than Vchg such that the pass transistor""s base-emitter junction blocks current from a battery connected to the battery connection terminal from flowing through the pass transistor when the charger is not in use. Thus, when the system is not charging or the charging supply is low or not present, the PNP acts as a blocking diode to prevent the battery voltage from appearing on the charger supply side of the pass transistor and providing bias to the charger controller circuitry.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.